1. Field of the Invention:
This invention relates to a metal-made carrier body adapted to carry thereon an exhaust gas cleaning catalyst which is generally provided as an exhaust gas cleaning means at an intermediate point of an exhaust pipe of an automobile.
More specifically, this invention makes use of a double-layer structure as a metal material of a cylindrical metal casing of the metal-made carrier body, thereby avoiding breakage and cracking of sheet-like band and corrugated band, said bands making up the metal-made carrier body, separation between a multi-layered composite body of these bands and the metal casing, corrosion of an outer surface of the metal casing, etc. and hence providing a metal-made carrier body capable of fully performing its function even under severe use conditions.
2. Description of the Related Art:
Conventional metal-made carrier bodies of the above sort, which are adapted to carry an exhaust gas cleaning catalyst thereon, include those having a structure formed by laminating at least one sheet-like metal band and at least one corrugated metal band in layers or rolling them together into a multi-layered composite body with a view toward increasing the carrying area per unit volume, namely, with a view toward increasing as much as possible the effective area of contact between exhaust gas and the exhaust gas cleaning catalyst per unit volume and further reducing the own weight of the metal-made carrier body to a maximum possible extent.
For example, a sheet-like metal band made of a heat-resistant thin metal sheet having a thickness of 0.1 mm or smaller and a corrugated band made from another thin metal sheet of the same type are superposed one over the other to have areas of contact therebetween. They are then rolled together spirally into a multi-layered composite body defining many network-patterned gas flow passages along a central axis thereof for allowing exhaust gas to pass therethrough. The multi-layered composite body thus rolled is enclosed within a cylindrical metal casing which has a single-layer structure and opens in both ends thereof. The members of the multi-layered composite body, i.e., the sheet-like band and corrugated band are put together into a vibration-proof structure. Namely, the sheet-like band and corrugated band as well as the multi-layered composite body thus rolled and the metal casing are put together at the areas of contact therebetween by welding, brazing or the like.
As the heat-resistant thin metal sheets for making the multi-layered composite body, metal sheets of ferritic stainless steel having a small coefficient of thermal expansion such as chromium steel (Cr content: 11-13.5%) or chromium-aluminum steel (Cr content: 20%, Al content: 5%) can be used. On the other hand, as the material for the metal casing, a single-layer austenitic stainless steel sheet is used to meet the requirements for high-temperature mechanical strength and corrosion resistance to the external atmosphere although it has a large coefficient of thermal expansion.
The joining of the sheet-like band and corrugated band, which make up the multi-layered composite body, and that of the multi-layered composite body and the metal casing require brazing with a high-temperature brazing material such as heat-resistant Ni because of the high temperature of the exhaust system of an automobile. Upon brazing, the metal casing made of the austenitic stainless steel having the large coefficient of thermal expansion thus undergoes substantial thermal expansion while the multi-layered composite body made of the ferritic stainless steel having the small coefficient of thermal expansion does not expand significantly. Accordingly, a thermal stress occurs at each area of contact so that the associated members may be separated there. In addition, when a metal-made carrier body formed by enclosing the multi-layered composite body within the metal casing is actually installed in the exhaust system of an automobile, the members making up the metal-made carrier body are subjected to repeated thermal stresses under the cooling and heating cycle of from -20.degree. C. to 900.degree. C. because strains (thermal strains) occur around the areas of contact between the multi-layered composite body and metal casing because of the large difference between their coefficients of thermal expansion. In addition, stresses caused by external vibrations such as those transmitted from an associated engine are also applied, so that the areas of contact between the multi-layered composite body and metal casing are subjected to substantial fatigue stresses. As a result, the members making up the metal-made carrier body, namely, the multi-layered composite body and metal casing show greater tendency of separation around the areas of contact therebetween, and at the same time the members forming the multi-layered composite body, i.e., the sheet-like band and corrugated band also become more susceptible to cracking and breakage. The conventional metal-made carrier bodies however cannot prevent such fatigue failure.